1. Field of the Invention
The invention relates to control apparatus and method for a vehicle having a driving mechanism in which a continuously variable transmission (hereinafter called “CVT”) is coupled to the output side of an internal combustion engine, such as a diesel engine. More particularly, the invention is concerned with control apparatus and method for controlling an output of the internal combustion engine so as to reduce the amount of a pollutant(s) in exhaust gas without deteriorating the fuel economy. The invention also relates to control apparatus and method for an internal combustion engine, such as a diesel engine, and more particularly to control apparatus and method for the internal combustion engine having a catalyst for purifying exhaust gas in an exhaust system.
2. Description of Related Art
An internal combustion engine, such as a diesel engine, generates motive power by burning a fuel, and therefore inevitably emits or discharges exhaust gas. In view of global environmental protection, for example, it is desired to purify exhaust gas emitted from the internal combustion engine as much as possible. Examples of pollutants contained in the exhaust gas from internal combustion engine include nitrogen oxides (NOx), hydrocarbon, and particulate matters (PM), such as smoke. To purify the exhaust gas, it is required to reduce the amounts of these pollutants.
NOx is likely to be generated when fuel is burned at a relatively high temperature in an oxidizing atmosphere. This means that NOx is easily generated when the air-fuel ratio (i.e. the ratio of air to fuel in an air-fuel mixture which is burned in the internal combustion engine) is greater than the stoichiometric air-fuel ratio (14.5) and is also in the proximity of that value (i.e. when the air-fuel ratio is in a range of about 16 to 17). Therefore, the air-fuel ratio may be set lower or greater than this range in order to reduce the amount of NOx. However, reducing the air-fuel ratio causes an increase in the amount of fuel supply, resulting in increased fuel consumption or deteriorated fuel economy. Also, increasing the air-fuel ratio makes combustion unstable depending upon the degree of the increase, thereby deteriorating fuel economy. Thus, a fuel consumption characteristic (or fuel economy) and a NOx emission characteristic have a contradictory relationship with each other, which means that an improvement of one of these characteristic results in deterioration of the other characteristic. Such a contradictory relationship is also established between NOx and particulate matters (PM). Namely, if the amount of one of these emissions is reduced excessively, the amount of the other emission increases significantly.
In view of the above-described situation, attempts have been made to achieve good characteristics in terms of both fuel economy and NOx emission, while focusing on the fact that the revolution speed of the internal combustion engine can be controlled as desired to some extent by connecting the CVT to the output side of the internal combustion engine. One example of such attempts is disclosed in Japanese Laid-Open Patent Publication No. 4-255541. In an apparatus disclosed in this publication, fuel consumption characteristics and NOx emission characteristics are obtained with respect to each of a rich operating state in which the air-fuel ratio is set at or smaller (richer) than the stoichiometric air-fuel ratio and a lean operating state in which the air-fuel ratio is set larger (leaner) than the stoichiometric air-fuel ratio. The apparatus then evaluates fuel consumption and NOx emission characteristics with respect to an operating state for obtaining an output that is based on a running condition of the vehicle and a required driving amount, and selects a suitable operating state that satisfies both of desired fuel consumption characteristic and NOx emission characteristic.
The control apparatus as described in the aforementioned publication is able to evaluate which one of lean-burn operation and stoichiometric air-fuel ratio operation (stiochiometric operation) achieves both of desired fuel consumption rate and desired NOx emission rate on the equi-output line corresponding to an actual output, and select an appropriate operating state that achieves better results in terms of fuel consumption and NOx emission. Although this arrangement gives grounds for selecting a lean-burn operation or a stoichiometric operation on the equi-output line, it is not designed to determine the optimal operating state. Namely, in the case where the fuel consumption rate and the amount of NOx emission change with the engine speed and the engine torque, the apparatus is not able to determine the optimal operating point that minimizes both the fuel consumption rate and the amount of NOx emission, and is thus not necessarily able to satisfy practical needs or requirements.
Also, in recent years, regulations for emission of environmental pollutants, such as NOx, are getting much stricter, and, as stated in the aforementioned publication, it is getting difficult to comply with the current emission regulations simply by changing operation conditions or combustion conditions of an internal combustion engine. In order to comply with these stringent NOx regulations, attempts have been made to control an operating state or conditions of the vehicle so as to achieve good characteristics in terms of both the fuel economy and the NOx emission, and also to purify exhaust gas from the internal combustion engine by using catalyst or trapping PM with a filter provided in an exhaust channel.
A NOx storage-reduction catalyst is known as an example of the catalyst. This catalyst absorbs NOx, as nitrogen in the form of a nitrate, contained in exhaust gas generated, for example, when the internal combustion engine is operated in a lean-burn condition with a relatively large air-fuel ratio. With the amount of the absorbed NOx increased to a predetermined amount, the catalyst is exposed to a reducing atmosphere for reaction, so that the stored nitrate-form nitrogen is reduced and released as nitrogen gas. In such a case, oxygen (active oxygen) in a nascent state is generated, thus enabling oxidization of soot attached to the catalyst.
When using this type of catalyst, the atmosphere to which the catalyst is exposed needs to be temporarily controlled to a reducing atmosphere at the time when the amount of NOx stored is increased to a certain level. Known control methods for creating the reducing atmosphere include (a) supplying a reducing agent, such as a fuel or ammonia, into exhaust gas, and (b) reducing the air-fuel ratio by increasing the amount of fuel supplied to the internal combustion engine. Since it is undesirable that ammonia be released from vehicles without reacting with other substance, fuel is normally used as the reducing agent. Thus, when the aforementioned NOx storage-reduction catalyst is used, a certain amount of fuel is consumed to reduce and discharge the NOx stored in the catalyst.
As described above, when the NOx storage-reduction catalyst is used, fuel is consumed for combustion in the internal combustion engine and also for removal of NOx. However, since the known control only takes account of the amount of fuel burned in the internal combustion engine, a further improvement to the known control is to be made so as to improve the fuel economy. Although the apparatus as disclosed in the above-identified publication gives grounds for selecting either a lean-burn operation or a stoichiometric operation on the equi-output line, it is not designed to determine the optimal operating state while taking account of the amount of fuel consumed for removing pollutants, such as NOx, in the exhaust gas. Accordingly, the fuel efficiency or fuel economy is not necessarily optimized when the NOx storage-reduction catalyst is used.
In addition, a so-called emission control device including the filter or the catalyst as mentioned above is not necessarily able to purify exhaust gas without limitation, but its function or activity needs to be restored or recovered. Also, the operation of the internal combustion engine may be influenced by the continuous operating time up to a point of time when a process of restoring the function or activity is required, or the content of the recovery process. It is thus necessary to satisfy both technical requirements, such as fuel economy, for engine operations and requirements for purifying exhaust gas. Nevertheless, effective devices or techniques for achieving both requirements have not been sufficiently developed, nor disclosed in the aforementioned publication.